Automatic friction shoes have long been desirable for window sash counterbalance systems. This is because the spring forces used in most counterbalance systems vary with the position of the sash, while the weight of the sash remains fixed, resulting in overbalance and underbalance at some sash positions. This would cause hop or drop, except for the resistance of the frictional forces involved. Of these, the friction between the sash and the jamb liner is generally constant, but the friction of the counterbalance system itself can be varied. This has led to the recognition of automatically variable friction produced by the counterbalance shoes, which interconnect the sash and the counterbalance system. The automatic variation of this friction is preferably responsive to the counterbalance forces and the sash weight, to produce more friction for heavier sashes and stronger counterbalances.
Automatic shoe friction can be produced in many ways, but a common preference is to take advantage of the offset that normally exists between the upward force of a counterbalance element applied to the shoe and the downward force of the weight of a sash applied to a platform extending out from the shoe. Since these upward and downward forces are ordinarily offset from each other, they produce a moment arm that tilts or torques the shoe so that upper and lower surfaces of the shoe rub frictionally against inside surfaces of the jamb liner channel in which the shoe runs. The automatic friction produced by this can be adjusted by the length of the offset moment arm, the vertical height of the shoe, the materials used for the shoe body and the jamb liner channel, and the configuration of the shoe surfaces rubbing against the jamb liner.
The prior art contains many variations applying these operating principles to automatic friction shoes, and two recent examples (U.S. Pat. Nos. 5,036,622 and 5,117,586) are relevant for suggesting that the sash platforms for such frictional balance shoes can be as simple as bent wires. These suffer some disadvantages, though, including pivoting of the wire from vertical, which can cause it to rub against the side of the slot in the jamb liner, and related problems involving mounting the wire reliably on the shoe body so that it affords a secure grip on the supported sash. This invention improves on the sash platform, to solve the problems encountered by bent wire sash platforms, and to accomplish all the functions necessary to an automatic friction shoe, effectively and reliably at a low cost.